Mechanisms of visual transduction and specific ionic membrane mechanisms are being studied in large photoreceptors of the barnacle B. eburneus and in the giant cell of the abdominal ganglion of Aplysia californica. Techniques involved are: a) voltage clamp, b) ion-specific electrodes, c) intracellular injection, d) spectrophotometry of pigment changes, and e) other optical probes of events involved in transduction. Studies are being conducted on 1) membrane latch-up by light, 2) pH and CO2, 3) changes in intracellular activities of Na, K and Cl associated with changes in the external ionic environment, 4) permeability sequences and ratios of mono and divalent cations in different adaptation states, and 5) electrical correlates of ion transport mechanisms.